New approach to invisibility cloaking gets much closer to the science-fiction version

Jan 25, 2011 by David L. Chandler
This image shows a calcite crystal laid upon a paper, causing all the letters to show double refraction.

The idea of being able to become invisible, especially by simply covering up a person or an object with a special cloak, has a perennial appeal in science-fiction and fantasy literature. In recent years, researchers have found ways to make very exotic "metamaterials" that can perform a very crude version of this trick, keeping an object from being detected by a certain specific frequency of radiation, such as microwaves, and only working at microscopic scales. But a system that works in ordinary visible light and for objects big enough to be seen with the naked eye has remained elusive.

Now, a team of researchers in the Singapore-MIT Alliance for Research and Technology (SMART) Centre has found a relatively simple, inexpensive system that can hide an object as big as a peppercorn from view in ordinary . The team’s discovery has been published online in Physical Review Letters and will appear soon in the print version of the journal.

Unlike the other attempts to produce invisibility by constructing synthetic layered materials, the new method uses an ordinary, common mineral called calcite — a crystalline form of calcium carbonate, the main ingredient in seashells. “Very often, the obvious solution is just sitting there,” says MIT mechanical-engineering professor George Barbastathis, one of the new report’s co-authors.

The paper was co-authored by SMART postdoctoral fellow Baile Zhang, MIT postdoctoral fellow Yuan Luo, and SMART researcher Xiaogang Liu, and the research was funded by Singapore’s National Research Foundation (NRF) and the U.S. National Institutes of Health (NIH).

In the experiment reported in this paper, the system works in a very carefully controlled setting: The object to be hidden (a metal wedge in the experiment, or anything smaller than it) is placed on a flat, horizontal mirror, and a layer of calcite crystal — made up of two pieces with opposite crystal orientations, glued together — is placed on top of it. When illuminated by visible light and viewed from a certain direction, the object under the calcite layer “disappears,” and the observer sees the scene as if there was nothing at all on top of the mirror.

For their demonstration, they placed the MIT logo upside-down on the vertical wall behind the apparatus, placed so that one of the letters could be viewed directly via the mirror, while the other two were behind the area with a 2-millimeter-high wedge (the height of a peppercorn) and its concealing layer of calcite. Then, the whole setup was submerged in liquid. They showed that the logo appeared normal, as though there was no wedge but a flat mirror piece, when illuminated with visible green light. Any imperfection in the cloaking effect would have shown up as a misalignment of the letters, but there was no such anomaly; thus, the cloaking operation was proven. With blue or red illumination, the cloaking was still effective but with some slight misalignment.

Calcite has long been known to have unique optical properties, including the ability to bend (or refract) a ray of light differently depending on the light’s polarization (the orientation of its electric field); these properties can cause the phenomenon of double refraction, or seeing “doubles” when looking through calcite with regular unpolarized light. In this research, the two pieces of calcite were oriented to bend the light in such a way that the emerging beam, after going through multiple reflections and refractions, appeared to be coming directly from the original mirror at the base of the setup, rather than from the actual higher point above the hidden object. The total optical path was also preserved, which means no scientific optical instrument can possibly uncover the cloaked wedge.

Submerging the apparatus in a liquid with a carefully chosen degree of refraction preserves the illusion. Barbastathis says the setup would work without the liquid, but the light’s transition into air would cause some blurring that would make the effect less convincing.

In principle, Barbastathis says, the same method could be used in real-life situations to conceal an object from view — and the only limitation on the size of the hidden object is the size of the calcite crystal that’s available. The team paid about $1,000 for the small crystal it used, he says, but much larger ones could be used to conceal much larger objects. (The largest known natural crystal of calcite measures 7 by 7 meters, or more than 21 feet across).

For now, the system is essentially two-dimensional, limiting the cloaking effect to a narrow range of angles; outside these angles, the cloaked object is quite visible. “We do have some ideas for how to make it fully three-dimensional,” says Barbastathis, the Singapore Research Professor of Optics and Professor of Mechanical Engineering. In addition, the team would like to eliminate the need for immersing the system in liquid and make it work in air.

Aside from its obvious potential applications in defense or law enforcement, the ability to render something invisible could have uses in research, Barbastathis suggests, such as providing a way to monitor animal behavior without any visible distraction. “The important thing is that now this is out in the open, people will start to think about” how it might be used.

Coincidentally, another independent research team, based at the University of Birmingham in the UK, has also published a paper this month describing a similar method for achieving a visible-light using calcite.

The MIT and Birmingham results “are two beautiful experiments. I particularly like their simplicity,” says Ulf Leonhardt, chair in theoretical physics at Scotland’s University of St. Andrews, who was the author of one of the first papers that described a metamaterial-based cloaking system. “Cloaking has been inspired by research on metamaterials,” he adds, “but, ironically, these cloaking devices are almost ‘home-made.’ Instead of sophisticated optical that are difficult to make and have many problems of their own, they use simple calcite crystals.”

Compared to the earlier versions of cloaking systems that only worked for microscopic objects, and only when viewed using radio or infrared wavelengths, the new approach is “closer to science fiction,” Barbastathis says. “Science is usually a bit disappointing when you compare it to sci-fi,” he says, but in this case “we came one step closer” to the imaginative vision.


This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

Explore further: Serial time-encoded amplified microscopy for ultrafast imaging based on multi-wavelength laser

More information: prl.aps.org/abstract/PRL/v106/i3/e033901

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kevinrtrs
1 / 5 (9) Jan 25, 2011
Exciting stuff this! Of course the multi-billion drug industry will not be sitting still with regards to this. They have just as much to conceal as the military and especially FROM them.
We have the usual case- whatever is discovered can be used for good or evil.
Now where did I leave that map of calcite deposits...?? It was here a few moments ago.
Telekinetic
1 / 5 (12) Jan 25, 2011
"If you can imagine it, it will come to pass. Invisibility is around the corner, and anti-gravity is next."- Telekinetic (1/23/11)
Ye of little faith.
Physmet
not rated yet Jan 25, 2011
I imagine they must be studying the properties of calcite so that they can make a better manufactured material that will drive down cost, allow flexibility in design, and hopefully get around the liquid/air problem. Maybe invisibility will be possible some day after all!
dinkster
not rated yet Jan 26, 2011
I want to see a picture. You might not see the peppercorn, but you sure can see the calcite. No pictures on the source site either.
poof
5 / 5 (4) Jan 26, 2011
Did that asshole 3 posts above just quote himself?

Ok, so they discovered polarization. Someone hand these people a nobel.
Telekinetic
1 / 5 (9) Jan 26, 2011
Asshole, eh? Coincidentally, your mother was just as
uninterested when she ejected you as you are with this
stunning advance. Here's another prediction, one of which I'm positive; you have never had or will ever have an original thought or idea.
Skeptic_Heretic
not rated yet Jan 26, 2011
Asshole, eh? Coincidentally, your mother was just as
uninterested when she ejected you as you are with this
stunning advance. Here's another prediction, one of which I'm positive; you have never had or will ever have an original thought or idea.
He probably won't pretentiously quote himself on the internet either.
trekgeek1
5 / 5 (1) Jan 26, 2011
"I'll be more impressed when I can wear an invisibility cloak like Harry Potter."
-Trekgeek1 (1/26/11)

That's what we're doing now, right?
Telekinetic
1 / 5 (8) Jan 26, 2011
Telekinetic - Nov 02, 2010
"What lab are you affiliated with, Skeptic?"

Skeptic_Heretic - Nov 02, 2010
"I'm a freelance metrology contractor. I deal with high end measurement apparatus for multiple labs, primarily medical. Why do you ask?"

Pretentious, perhaps, but I'm not a pretentious liar.
Quantum_Conundrum
1 / 5 (4) Jan 26, 2011
Ah yeah, Jesse Ventura and "The Rock" did it too.
AgentOfFate
1 / 5 (3) Jan 26, 2011
Until I have a witty quote of my own, this is just plain cool.
xamien
not rated yet Feb 01, 2011
Darn it. I wanna have a cool quote, too.